An electric motor of a permanent magnet tangential-magnetization structure can generate a higher air gap flux density than a permanent magnet radial-magnetization electric motor due to its magnetism gathering effect. As a result, the electric motor has a relatively large torque/current ratio and torque/volume ratio and is increasingly applied to an occasion such as a servo system, electric traction, office automation and a household electrical appliance.
In a tangential permanent magnet electric motor, due to a magnetic circuit structure in which single permanent magnets are in parallel connection, operating points of the permanent magnets are lower than a radial magnetic electric motor. Moreover, most permanent magnets are of a rectangular shape, and widths thereof along a radial direction of a rotor are almost the same. However, the intensities of a demagnetized field on different parts of the permanent magnets are different and this is the case for the operating points on different parts of the same permanent magnet. Hence, the overall demagnetization resistance of the electric motor is reduced. Particularly, the closer the permanent magnets are to an outer side of the rotor, the stronger the intensities of the demagnetized field are borne, such that an efficiency of the electric motor is reduced.
Therefore, how to reduce the local demagnetization of the permanent magnets and ensure the efficiency of the electric motor is a technical problem to be solved by a person skilled in the art at present.